1. Field of the Invention
The present invention generally relates to a computerized tomographic (CT) imaging method and a CT imaging system capable of helically dynamic-scanning a biological body under medical examination to obtain a CT image thereof. More specifically, the present invention is directed to such CT imaging method/system capable of helically scanning the same biological body for several times, while a contrast medium is injected into this biological body, thereby obtaining a contrast image thereof without any artifact.
2. Description of the Prior Art
Various types of CT (computerized tomographic) imaging methods/systems have been developed in the medical electronic fields, for instance, X-ray CT imaging systems, single photon emission CT (SPECT) imaging systems, and position emission tomography (PET) imaging systems.
In particular, a so-called "helical scanning" type X-ray CT imaging apparatus/system has also been developed and marketed in this field. A typical "helical scanning" type X-ray CT imaging apparatus is known from, for instance, U.S. Pat. No. 4,630,202 to Isei Mori, entitled "COMPUTERIZED TOMOGRAPHIC APPARATUS UTILIZING A RADIATION SOURCE", patented on Dec. 16, 1986. In this helical scanning type X-ray CT apparatus, while a biological body under medical examination is translated with respect to an X-ray source and an X-ray detector, a predetermined portion of this biological body is scanned as a data acquisition region by projecting X-ray beams through this portion to the X-ray detector in such a way that the X-ray source is continuously moved along a helical orbit around this biological body. For a further description of helical scanning techniques, see the above-described Mori U.S. Patent specification.
Helical scanning type CT imaging method/system defined in the improvement of the present invention provides improvement over those described above. However, to facilitate understanding of the present invention, one conventional helical scanning type X-ray CT imaging system will now be described.
Referring now to FIGS. 1 to 7, the typical helical scanning operation and the artifact problem caused in one conventional X-ray CT imaging system will be explained. FIG. 1 schematically shows an overall arrangement of the conventional X-ray CT imaging system. FIGS. 2A and 2B schematically represent helical dynamic scanning operation timing charts of the conventional X-ray CT imaging system. FIGS. 3A and 3B illustrate angiograms and a functional image acquired by the conventional X-ray CT imaging system. FIG. 4 schematically indicates a detector array of the conventional nutate-rotate type X-ray CT imaging system. FIG. 5 schematically indicates a basic structure of this conventional helical dynamic scanning type X-ray CT system. FIGS. 6A, 6B, 6C and 7 schematically show the couch position/X-ray source angle/helical scanning orbit of the conventional helical scanning type X-ray CT imaging system.
Referring back to FIG. 1, the overall arrangement of the conventional helical scanning type X-ray CT imaging system will now be described.
In FIG. 1, a pair of X-ray source 41 and X-ray detector 31 are positioned within a gantry 2 in such a manner that the X-ray source 41 and the X-ray detector 31 are mutually rotatable and positioned opposite to each other with respect to a biological body 10 under medical examination laid on a couch 43. The couch 43, i.e., the biological body 10 such as a patient is translated along a direction indicated by symbol "Z" (namely, longitudinal direction of the biological body) by driving a couch servomotor 40. A present position of this couch 43 is sensed by a couch position sensor 45.
The X-ray source 41 and the X-ray detector 31 are relatively rotatable by driving a detector servomotor 20 along a rotation direction "R". A rotation angle of this X-ray detector 31 is sensed by an angular sensor 30 to produce angle data ".theta.". A data acquisition unit 3 is employed within the gantry 2 to produce helical scanning data.
A main control unit 50 is employed and a clock generator 52 is also employed. In response to clock pulses produced from the clock generator 52, the main control unit 50 mainly supplies control signals to the DAS unit 3, the X-ray detector servomotor 20, the angular sensor 30, the couch servomotor 40, and a computing unit 80. A CT image derived from the computing unit 80 is displayed on a monitor 82.
Referring now to the timing charts shown in FIGS. 2A, 2B and the images indicated in FIGS. 3A and 3B, the conventional helical dynamic scanning operation by the X-ray CT imaging system of FIG. 1 will be described.
In FIG. 2A, an ordinate represents a slice position of the conventional dynamic scanning operation, and an abscissa shows time instants "t.sub.A0 ", "t.sub.A1 ", - - - , "t.sub.B0 ", "t.sub.B1 ", - - - , "t.sub.02 ". Symbols "A.sub.0 ", "A.sub.1 ". - - - , "B.sub.0 ", "B.sub.1 ", - - - , "B.sub.2 " represent X-ray images acquired at the above-described time instants "t.sub.A0 ", - - - , "t.sub.03 ", respectively.
Assuming now that an X-ray contrast medium is injected to the biological body 10 under medical examination at a time instant between the time instant "t.sub.B0 " and the time instant "t.sub.A1 ", since two sets of images "A.sub.0 " and "B.sub.0 " correspond to images acquired before the injection of the X-ray contrast medium, as shown in FIG. 2B, subtraction images "A1-A0", "B1-B0", "A2-A0", "B2-B0", "A3-A0", and "B3-B0" are formed in the computing unit 80. FIG. 3A schematically shows subtraction images "A1-A0", "A2-A0", and "A3-A0". A functional image may be obtained from these subtraction images "A4-A0", "A2-A0", and "A3-A0" by extracting characteristic values of variations contained in the images and then by indicating the characteristic values as high/low density values. For instance, FIG. 3B schematically shows a peak time image (=functional image) the respective pixels of which are indicated by the high/low density values corresponding to such time instants when the density values of the X-ray contrast become maximum, namely the CT values thereof become maximum. From such a functional image shown in FIG. 3B, it can be easily recognized such a position where the X-ray contrast medium has reached at first.
As previously described, when the helical scanning operation would be repeatedly performed for the same slice portions of the biological body 10, while the X-ray contrast medium is injected into this biological body (namely, the helical dynamic scanning operation is carried out), an artifact would be practically induced in the subtraction images of FIG. 3A.